This pH and pOH calculator turns a hydrogen ion concentration into the full picture of how acidic or alkaline a solution is at room temperature. The pH scale measures acidity using a logarithm, defined as pH equals the negative base ten logarithm of the hydrogen ion concentration in moles per litre. Because water self ionises, pH and pOH are linked by the simple relationship pH plus pOH equals 14 at 25 degrees Celsius, which lets you find one from the other. You enter the hydrogen ion concentration in moles per litre, and the tool returns the pH, the matching pOH, and the hydroxide ion concentration worked back from pOH. A pH below 7 is acidic, a pH of 7 is neutral, and a pH above 7 is alkaline or basic. Chemistry students, lab technicians, water testers and gardeners use this tool to interpret titrations, buffer solutions, soil samples and aquarium readings without reaching for a calculator and log tables. To get reliable answers, keep your concentration in moles per litre and use scientific notation for very small numbers, since values like 0.000001 are easy to mistype. Remember that the 14 relationship assumes a temperature of 25 degrees Celsius, because the ion product of water changes with temperature and shifts neutral pH slightly at other temperatures. It also helps to sanity check your result against the scale, since a strong acid should give a low single digit pH and a strong base should give a high one, so a surprising answer usually means a stray zero in the concentration you entered.
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The pH is the negative base ten logarithm of the hydrogen ion concentration, so a concentration of 0.001 moles per litre gives a pH of 3. The pOH is 14 minus the pH, which is 11. The hydroxide concentration is ten raised to the power of minus the pOH.
Worked example
With a hydrogen ion concentration of 0.001 moles per litre, the pH is the negative log of 0.001, which is 3.00. The pOH is 14 minus 3, which is 11.00, and the hydroxide concentration is ten to the power minus 11, written as 1.00e-11 moles per litre.
